A filtration unit having hydrophilic, porous polymeric membranes and which is capable of being tested for integrity finds application in areas with high demands for filtration reliability, i.e., in the pharmaceutical and food industries, in medicine, in laboratory work and in biotechnology. Porous membranes are rendered hydrophilic either by virtue of their structural morphology or by the addition of a hydrophilic wetting agent or other hydrophilic additive. Such filters are normally used in the form of modules or cartridges.
Filtration reliability is understood in the art to mean that the filtration unit must show sufficiently high mechanical and chemical stability to last a reasonable period of time and must be capable of being tested for integrity. Filtration capacity relates to the volume flow of filtrate per unit of time, which is typically expressed as flux.
A frequently employed integrity test for filtration units having hydrophilic, porous membranes is that constituting fluid impingement on one side of the membrane, the pores of which are filled with a fluid such as water or by a gas such as air under a test pressure. From the measurement of the duration of the pressure loss at the membrane side pressurized with test gas, or by the quantity of the test gas which penetrates through the membrane, conclusions can be drawn as to the integrity of the membrane within the filtration unit. Exemplary tests include burst tests, bubble-point tests and diffusion tests. All known integrity tests for hydrophilic membranes require a thorough wetting of both the membrane filtration surface and its peripheral sealed surfaces.
In general, the use of hydrophilic, porous membranes in filtration units is expensive, if one wishes to maintain a relatively small inventory of filtration units that are not integrity testable. A current method of incorporating such membranes into hydrophilic filtration modules comprises potting such membranes at their peripheries in thermoplastic resin end caps or anchoring elements so as to achieve a fluid-tight seal between the membrane and the module at both ends of the module. The chief drawback of this known method is that the heat from the melt of the thermoplastic resin tends to denature the hydrophilic, porous membranes in the area of contact with the membranes and in areas adjacent thereto, often rendering the membranes hydrophobic in those areas. As a consequence, these hydrophobic-rendered peripheral areas can no longer be wetted with water, which in turn causes the integrity test gas to pass without hindrance through the pores in these areas, thereby simulating a failed membrane.
In addition, such hydrophilic membranes tend to be brittle, and so can be only lightly stressed by mechanical loading, and are susceptible to tearing and tend toward progressive fissuring. Thus, in the treatment of these membranes, for instance where cutting, stamping or pleating are concerned, or even in the act of incorporation into a module, defects can be introduced. This already fragile mechanical stability of the membrane may be further impaired by the action of the hot melt of the thermoplastic resin during the potting, often leading to breaks in the membrane, with the result that the filtration modules are no longer testable for integrity. In biological fluid filtration applications, this is unacceptable because of the risk of contaminants bypassing the membrane filter and, for example, entering the blood stream of a patient:.
One way to cure such problems is disclosed in PCT Application No. 96/14913 wherein the resulting peripheral hydrophobic areas are post-treated with a hydrophilic agent. Several suggestions for overcoming the hydrophobic periphery problem are disclosed in European Patent No. 0 096 306. One proposal is to seal the edges of the hydrophilic membrane with non-porous polyester film, which on one side, is provided with a solvent-free polyethylene coating serving as a fusion adhesive. In the same patent, the reduction of the porosity of the hydrophilic membrane along the membrane edges was also proposed by the use of a casting dope which yielded a smaller pore size than that of the main surface of the membrane. A third method was to mechanically compress the membrane along its edges so as to collapse its pores in the compressed areas.
European Patent No. 0 036 315 also discloses a mechanical procedure, whereby the sensitive peripheral area of the hydrophilic membranes is treated by applying an adhesive thereto. The disadvantage of this treatment lies in the unsatisfactory temperature stability of the membrane areas which have been so treated, when they are exposed to a plurality of steam cycles at 134.degree. C. Commonly assigned U.S. Pat. No. 4,969,997 describes a method of imparting fluid impermeability to the edges of porous membranes by forming a film over the surface of the membrane edges, the film formation being accomplished by contacting the membrane edges with a hot gas to liquify the membrane in the edge area, then allowing the same to cool so as to seal the pores and form a film.
German Patent No. 43 39 810 discloses impregnation of the peripheral areas of the membranes with a hydrophilic-rendering agent to thereby saturate the membranes in those areas. The so-treated membranes are subsequently washed and dried. Theoretically, these membranes should possess, in their impregnated areas, at least double the hydrophilicity as the untreated areas. The result of this is that following potting of the membranes, because of the great surplus of hydrophilic agent in the peripheral areas of the membranes, no edge hydrophobic characteristics appear.
All of the above proposed solutions possess at least one of the following disadvantages:
they are technologically expensive in money and time, PA1 they introduce foreign agents which may be flushed out during filtration, PA1 the active filtration surface is diminished by film deposition or by coating with a non-porous film, thereby reducing flux, and PA1 the peripheral areas of the membrane suffer from mechanical injury.
Thus, the purpose of the present invention is to create a hydrophilic membrane filter unit capable of being tested for integrity, which, in the periphery of the membrane, is sealed with the thermoplastic resin-of a module without compromising the hydrophilicity in the peripheral or edge areas of the membrane and which has improved filtration reliability and capacity.